1. Field
Example embodiments relate to a fuse circuit. More particularly, embodiments relate to a fuse circuit having improved reliability and a semiconductor memory device including the fuse circuit.
2. Description of the Related Art
As the integration degree of a semiconductor memory increases, a capacity of the semiconductor memory is increasing rapidly. Increasing capacity includes increasing a number of memory cells in a chip. As the number of memory cells increases, a number of defective memory cells also increases. Since an existence of a single defective memory cell causes a critical problem in the semiconductor memory device, redundant memory cells are provided to compensate for the defective memory cells. Based on fuse information, redundant memory cells may replace defective memory cells. When an address of the defective memory cell is input, a normal path is cut off and a redundancy path to the redundant cell is activated according to programmed/unprogrammed state of the fuse.
A laser fuse, which is one of the widely used fuses, is programmed by cutting the fuse formed with a metal line using laser beam. The laser fuse has an advantage of being simply embodied, since an additional circuit to control cutting of the fuse is unnecessary. The laser fuse, however, has disadvantages, including limiting integration density regardless of the development of the semiconductor memory manufacturing process technology, e.g., metal oxide semiconductor (MOS) process, since a predetermined interval between the fuses has to be secured to prevent damages due to laser fusing. Furthermore, the laser fuse cannot be reprogrammed after the memory chip including the fuse circuit is packaged.
To overcome these disadvantages, an electrical fuse and an anti-fuse programmed using an electrical signal have been proposed. Such fuses have advantages that they can be reprogrammed even after the memory chip is packaged since they are activated or deactivated by the electrical signal. In addition, the size of the fuse circuit may be reduced as the scale of the manufacturing process is reduced.
A conventional fuse circuit includes a program unit including an electrical fuse, a reference voltage generator, and a comparator. The electrical fuse included in the program unit has a sufficiently smaller resistance than a resistance of the reference voltage generator before being programmed in a program mode, but has a larger resistance than the resistance of the reference voltage generator when the electrical fuse is cut by being programmed in the program mode, since the electrical fuse operates as an open circuit. The program unit and the reference voltage generator output respective voltages that are, in proportion to each resistance, pulled down from a power supply voltage. The comparator compares an output voltage of the program unit with an output voltage of the reference voltage generator and generates an output signal which indicates whether or not the electrical fuse is programmed, i.e., whether or not the electrical fuse is cut.
Another conventional fuse circuit includes a program unit including an anti-fuse, a reference voltage generator, and a comparator. The anti-fuse of the program unit has a sufficiently larger resistance than a resistance of the reference voltage generator before being programmed in a program mode, but has a smaller resistance than the resistance of the reference voltage generator when the program unit is programmed in the program mode, since the anti-fuse is electrically shorted. The program unit and the reference voltage generator output respective voltages that are, in proportion to each resistance, pulled down from a power supply voltage. The comparator compares an output voltage of the program unit with an output voltage of the reference voltage generator and generates an output signal which indicates whether or not the anti-fuse is programmed, i.e., whether or not the anti-fuse is electrically shorted.
The resistance of the program unit including the electrical fuse increases when the program unit is programmed and the output voltage of the program unit decreases. In contrast, the resistance of the program unit including the anti-fuse decreases when the program unit is programmed and the output voltage of the program unit increases. The output voltages of the program units change after being programmed. The comparators compare the reference voltage of the reference voltage generators with the output voltages of the program units, and determine whether or not the program units are programmed based on the comparison results.
As such, in the conventional fuse circuit, the reference voltage generators output a fixed reference voltage, and the comparators compare the fixed reference voltage with the output voltage of the program units including the electrical fuse or the anti-fuse and determine whether or not the program units are programmed. In the electrical fuse implementation, residues remain around the cutting area. Thus, the resistance of the program unit may not increase sufficiently. Such residues form an electrical path having a finite resistance. In the anti-fuse implementation, an electrical short may be incomplete. Thus, the resistance of the program unit may not decrease sufficiently. When the change of the resistance of the program units is insufficient, programming is incomplete and whether or not the program unit is programmed is indefinite, thereby degrading the reliability of the fuse circuit.
The electrical fuse is programmed (i.e. cut) by applying a relatively high current through the electrical fuse in response to a control signal. The electrical fuse can be cut by externally applying the control signal even after the fuse circuit is packaged. The electrical fuse, however, requires a large control driver for applying a high current through the electrical fuse. Thus, the size of the fuse circuit is relatively large.
The anti-fuse is programmed (i.e., electrically shorted) by applying a relatively high voltage to the anti-fuse in response to a control signal. Generally, the anti-fuse is a capacitor having both ends electrically connected when dielectric material in the capacitor is damaged by the applied high voltage. The anti-fuse can be programmed by externally applying the control signal even after the fuse circuit is packaged. The reliability of the anti-fuse is determined according to a level of the applied voltage. Thus, the anti-fuse needs high voltage for reliability.
As such, the electrical fuse and the anti-fuse have advantages, but also have a common disadvantage that their reliability is lower than the laser fuse, since electrical characteristic of such fuses may be varied according to the change of manufacturing process. To increase the reliability of the electrical fuse and the anti-fuse, various methods are proposed, but conventional methods can not guarantee the reliability of the fuses.